attempt to implement Fortinet challenge-based 2FA (ping #225)

attempt to implement Fortinet challenge-based 2FA (ping #225)

2FA involves reading a bunch of values from the initial response and
parroting them back, along with changing the 'credential' field to 'code'.

This is based on Openfortivpn's implementation
(https://github.com/adrienverge/openfortivpn/blob/master/src/http.c#L711-L754),
and has been tested for basic correctness against a toy HTTPS server that
gives responses with the right format.

Still to do:

- Another one-time password mode: https://github.com/adrienverge/openfortivpn/blob/master/src/http.c#L672-L679, https://github.com/adrienverge/openfortivpn/commit/61dd4fcbbfe1dd89b95081e7d5f5c3e933d7897a
- There's a mobile-app-based "push" mode: https://github.com/adrienverge/openfortivpn/commit/2f16d964560d9b00acbf7bfc7131a0ac40c688f2

Signed-off-by: Daniel Lenski <dlenski@gmail.com>